In this study, renal transplant patients are assigned into high and moderate potency anti-thymocyte globulin groups by a randomized double blind technique. Clinical parameters of patient and graft survival, rejection, infection and potential toxic side effects of these agents are compared in the two groups. In addition, a variety of cell-mediated immunity studies, including mixed lymphocyte culture, cell-mediated lympholysi, antibody-dependent-cell-mediated cytolysis, T cell reactivity (PHA and Con-A blastogenesis), and also T, B, and K cell levels, will be serially measured in these two groups and attempts made to correlate these in vitro results with in vivo immune reactiity. Techniques will be studied for detecting macrophage cytophilic antibody as well as serum blocking factors also, Overall, the proposed work is designed to better understand the allograft rejection phenomena and to attempt to develop more potent and selective immunosuppressive agents for organ transplantation in man. BIBLIOGRAPHIC REFERENCES; Thomas, J., Nelson, B,,Owens, L,, Thomas, F., et al. Correlation of In Vitro Immune Reactivity and Rejection in Human Transplant Recipients. Surg. Forum, 26:3l6, 1975. Thomas, F., Thomas, J., Mendez-Picon, G., et al.: Improved Results of Cadaver Transplantation in Immunologically High Risk Patients. Proceed. Am. Soc. Art. Int. Organs., 1976, In Press.